Unvented gas-fired burners are widely used as space heaters in dwellings and other buildings. Their thermal efficiency comes from their ability to reduce air infiltration rates, but they can be a source of indoor pollution especially in the amount of NO.sub.X formed particularly NO.sub.2.
NO.sub.X is a term used to describe the combine d "Oxides of Nitrogen" in particular NO, N.sub.2, and NO.sub.2. NO and N.sub.2 O for example are a concern in the outdoor environment, in particular with relation to acid rain, ozone and photochemical smog NO.sub.2, however, is of more concern to medical authorities due to the effect it has on lung function.
Medical research during the 1980's has suggested that much lower levels of NO.sub.2 will affect lung function than was previously thought. Until recently in New South Wales, Australia, for example, a 3 ppm upper limit per 8 hours of NO.sub.2 was considered safe and in the United States of America the figure was 5 ppm per 8 hours. However, the Public Health Committee of the National Health and Medical Research Council in Canberra, Australia, after considering all the new available medical data has decided that a level above 0.3 ppm give(s) reason for concern and the World Health Organization has now set a goal of 0.21 ppm (not to be exceeded for more than one hour per month).
Furthermore, in the outdoor environment general concern is increasing over the levels of NO.sub.X in both the lower and upper atmosphere and various authorities around the world are introducing legislation to control emissions in combustion products. These new targets and accompanying legislation are going to have a dramatic impact on the gas burner market as a whole.
Gas burners in general are of two types--the blue flame burner and surface combustion (radiant) burners. The type most commonly used in convection space heaters is the blue flame burner as they operate at a lower temperature than the surface combustion burners, making them safer for use in schools or the home. However, it is well established that blue flame burners generally produce NO.sub.2 in the levels in the order of 15 to 30 ng/Joule and as such are not considered to have potential for the reduction of NO.sub.X. For this reason, research into producing low burners has centered primarily around surface combustion burners of different forms.
In the last twenty years, research into the production of burners having lower NO.sub.X emission levels has concentrated on the use of excess air, alone or in combination with the incorporation of second stage burning. As a result, a number of these burners have become very complex in both design and operation procedures.
For example, the most successful to date have centered on using pressurized premixed air/flue mixtures burnt in a variety of metallic surface configurations, ceramic surfaces or after burners. All have relied on high excess air and high combustion load. In fact, burners with low combustion load were considered undesirable. Additionally, low pressure burners using high excess air while not using an air pump of some kind had not previously been considered acceptable, due to problems experienced with flashback.
Whilst reduction in NO.sub.X emission levels have been achieved relative to the older types of burners, it still appears that it has hitherto not been possible to even approach the new target levels considered desirable.